The present invention relates to the general technical field of producing a low-pressure plasma excited by microwave energy in the electron cyclotron resonance range.
The invention relates to all applications requiring controllable plasma uniformity over large areas that can be plane or curved in shape.
The present invention relates to a wide variety of applications, such as surface treatments, e.g. etching, deposition, chemical or thermochemical treatment, spraying, cleaning, disinfection, decontamination, or producing ion beams obtained by extracting plasma.
In the technical field of exciting a plasma to electron cyclotron resonance, resonance is obtained when the frequency of gyration of an electron in a static or quasi-static magnetic field is equal to the frequency of the applied accelerating electric field. This resonance is obtained for a magnetic field B at an excitation frequency f which are associated by the following relationship:
B=2xcfx80mf/e
where m and e are the mass and the charge of an electron.
When exciting a plasma, electron cyclotron resonance is effective only if the electron is sufficiently accelerated, i.e. if the electron can revolve in phase with the electric field for long enough to acquire the threshold energy necessary for ionizing the gas. To satisfy this condition, it is necessary firstly for the radius of gyration of the electron to be small enough, in particular to enable it to remain in that region of space in which resonance conditions exist, i.e. the simultaneous presence of the electric field and of the magnetic field, and secondly that the frequency of gyration f remains large relative to the frequency of collision between electrons and neutral elements such as atoms and/or molecules. In other words, the best conditions for exciting a plasma to electron cyclotron resonance are obtained when simultaneously gas pressure is relatively low and the electric field frequency f is high, which also means that the magnetic field intensity B must be high.
French patent No. 85 08 836 describes a technique for exciting a plasma to electron cyclotron resonance that requires permanent magnets to be used, each creating at least one surface at constant magnetic field and at an intensity corresponding to electron cyclotron resonance. Microwave energy is supplied to the resonance zone by antennas or plasma exciters each constituted by a metal wire element. Each exciter extends close to and along a magnet, being located in register with a permanent magnet.
The magnetic field of intensity equal to the value giving resonance and the microwave electric field are both located and confined essentially in the space situated between an exciter and the portion of the wall of the enclosure placed facing a magnet. In the presence of a gaseous medium at low pressure, electrons are accelerated in the resonance zone and wind along magnetic field lines which define a plasma confinement surface. The field lines form festoons connecting the pole of one magnet to its opposite pole or to the opposite pole of a consecutive magnet. Along its path, an electron dissociates and ionizes any molecules and atoms with which it comes into collusion. The plasma formed in this way in the festoons of the magnetic fields then diffuses away from the field lines to form a plasma that is practically free from the high energy electrons since they remain trapped in the festoons.
The major drawback of the technique described in that patent lies in the fact that the zone in which microwave energy propagates and the resonance zone in which the microwave energy is absorbed are superposed. Microwave propagation can therefore take place only with losses, and the intensity of the microwave electric field and the density of the plasma both diminish progressively along the exciter starting from the microwave source. The resulting plasma presents density that is not uniform along the exciter, and as a result such a plasma appears to be unsuitable for most industrial applications.
A variant of the above-described technique constitutes the subject matter of French patent No. 93 02 414. In that technique, the microwaves are applied directly to one of the ends of the magnetic field applicator constituted by permanent magnets or by a conductor carrying an electric current. In that case also, the microwave propagates mainly in the electron cyclotron resonance zone and are therefore subjected to a high level of attenuation, which leads to a plasma being produced that is not uniform along the magnet. The advantage of that solution compared with the preceding technique is firstly simplification of the apparatus, and secondly increased efficiency in plasma excitation insofar as the intensity of the magnetic field and the intensity of the microwave electric field are both at respective maxima at the surface of the applicator.
In order to remedy non-uniformity in the plasma associated with propagation of a highly attenuated traveling wave, French patent No. 91 00 894 proposes exciting the plasma by means of a standing wave of constant amplitude. To this end, it proposes causing microwaves to propagate in a zone that is distinct from that in which electron cyclotron resonance conditions are satisfied. The solution proposed is to apply the microwave power by means of a wire applicator situated, not facing the poles of the magnet, but halfway between two adjacent opposite poles in the zone that is forbidden to the plasma. Thus, it is possible to obtain a plasma that is uniform along the magnets, with the maxima and minima of the microwave fields due to the standing wave along the magnets being leveled out by the drift velocity of the electrons along the magnets, perpendicularly to the plane of the magnetic field.
Generalized use of standing waves for exciting uniform plasmas at electron cyclotron resonance is proposed in French patent No. 94 13 499 where the establishment of standing waves is controlled by adding wire propagators to the microwave applicator, which propagators are located outside the absorption zone for microwaves at electron cyclotron resonance.
Nevertheless, the techniques described in those patents No. 91 00 894 and 94 13 499 where microwave propagation takes place in the form of standing waves suffers from the difficulty of obtaining constant amplitude, in particular over a relatively long length of the microwave applicator. In addition, adjusting uniformity and amplitude generally requires impedance matching to be used upstream from the microwave applicator.
An analysis of the various prior art techniques leads to non-uniformities being observed in the electric field and in the plasma along the applicators, which non-uniformities are due to the very high attenuation of the electric field by resonant absorption of the microwave power by electrons at electron cyclotron resonance. In other words, the techniques described above do not enable a uniform plasma to be produced over a long length, and thus over a large area, typically of dimensions of meter order or greater. In addition, those techniques present a major drawback, that of microwaves being radiated from the applicators and propagators due to poor confinement of the microwave power in the propagation zone. This gives rise to coupling between the applicators that can be particularly harmful for obtaining good plasma uniformity. Another drawback lies in the difficulties of making non-rectilinear applicators or propagators and of inserting them into an evacuated enclosure. This makes it practically impossible to treat curved surfaces uniformly.
The object of the invention is thus to remedy the drawbacks of the prior art by proposing a method that makes it possible to produce a plasma that is uniform relative to a work surface of relatively large dimensions.
Another object of the invention is to provide a method of producing a plasma that is uniform relative to a work surface that is plane or curved in shape.
To achieve the various objects specified above, the present invention provides a method of producing a plasma for a work surface by means of apparatus comprising means for producing energy in the microwave range and means for creating at least one surface having a magnetic field that is constant and of intensity corresponding to electron cyclotron resonance, and that is placed at least in the microwave propagation zone for the purpose of plasma excitation.
According to the invention, the method consists:
in making a series of individual plasma excitation devices each constituted by a microwave energy wire applicator having one end connected to a source for producing microwave energy and having its opposite end fitted with at least one magnetic dipole as means for creating at least one surface having a magnetic field that is constant and of intensity corresponding to electron cyclotron resonance;
in mounting the magnetic dipole at the end of the microwave applicator in such a manner as to cause electrons accelerated to electron cyclotron resonance to oscillate between the poles so as to create a plasma diffusion zone situated at least on the side of the dipole that is remote from the end of the applicator; and
in distributing the individual plasma excitation devices relative to one another and in proximity relationship with the work surface in such a manner as to obtain at least juxtaposition between the plasmas produced by each of the individual devices so as to create a plasma that is uniform for the work surface.
In another aspect, the invention seeks to provide means for producing energy in the microwave range and means for creating at least one surface having a magnetic field that is constant and of intensity corresponding to electron cyclotron resonance, and that is placed at least in register with the microwave propagation zone in order to excite the plasma.
According to the invention, the apparatus comprises a series of individual plasma excitation devices each constituted by a microwave energy wire applicator having one end connected to a source for producing microwaves and whose other end is fitted with at least one magnetic dipole constituting means for creating at least one surface having a magnetic field that is constant and of intensity corresponding to electron cyclotron resonance, the dipole being mounted at the end of the microwave applicator so as to ensure that electrons accelerated to electron cyclotron resonance oscillate between the poles so as to create a plasma diffusion zone situated on the side of said dipole that is remote from the end of the applicator, the individual excitation devices being distributed relative to one another and in proximity with the work surface in such a manner as to create together a plasma that is uniform for the work surface.